The present invention is directed toward a filtering system and more particularly, toward a multi-stage cyclone filtering system for dry and wet air cleaning.
Cyclone systems for separating contaminants from the air are well known and widely used in various industries and household vacuum cleaning machines. They are described in technical literature and many patents, for example: Vinit Mody et al., Dust Control Handbook, p.65; Christie J. Geankoplis, Transport Processes and Unit Operation, pp. 838-840; U.S. Pat. No. 3,898,068; U.S. Pat. No. 5,228,890; and U.S. Pat. No. 5,236,479.
Typically, a cyclone system includes a cylindrical chamber usually mounted vertically or inclined. The chamber has a lid through which an exhaust nozzle for the exit of clean air extends. The nozzle is mounted in a coaxial fashion to the chamber. The cyclone separation of hard contaminants from the air occurs because of the differing densities of the contaminants and the air and the rotation of air flux. That is, heavy particles collect near the walls of the chamber and flow down to the bottom by gravity. The clean air collects in the center and is released from the chamber through the exhaust nozzle. Separated dust is periodically removed from the cyclone. The conical bottom of the cyclone is often connected, by a pipe, to a special box for collecting separated contaminants. The rotation of air flux in the cyclone takes place due to the tangential placement of the air feeding nozzle.
The above-described cyclone device, however, has an insuperable contradiction. On the one hand, increasing the speed of rotation of the air flux increases the centrifugal force acting on the air and contaminants which, in turn, improves the quality of separation of the contaminants from the air. On the other hand, the increase of the air flux speed increases the carryability of air, that is, the ability of the air to entrain and move the contaminant particles, which also decreases the quality of separation. See, for example, U.S. Pat. No. 3,898,068 to McNeil. This is why the cyclone cannot remove stream particles smaller than 5 .mu.m from the air. (Robert Noyes, Unit Operation in Environmental Engineering, p.321)
In order to increase the quality of separation, the contaminated air flux has to be rotated in the cyclone slowly but for a long time. Because air has a very low density and its inertia is very low, however, it quickly loses its initial direction of moving (rotation) after entering the cyclone. Its further movement is determined by the shortest way to the outlet. Because it becomes a straight line, the separation in this part of the air flux path stops. To increase the inertia of the air flux and the time of separation, the entering velocity is increased. For this reason, the diameter of the entering nozzle is decreased, compared to the diameter of the pipe conveying to the cyclone air flux. At the same time, however, as was mentioned above, the carry ability of the air increases and the quality of separation decreases.
There is also a method of improving the cleaning of air in the cyclone through the moistening of the contaminated air flow. See, Vinit Mody et al., Dust Control Handbook, p.81. According to this method, the dirt particles become heavy with moisture and attempt to collect into larger drops, which in turn, reduces the amount of centrifugal separation of dust from air. However, this phenomenon causes a new problem. The air must be then filtered out of the dispersed liquid. This is an especially difficult problem for household vacuum cleaners that use water to moisten the air flow. In such devices, two contradicting processes must take place in a compact space: the moistening of the contaminated air and the separation of air from the then contaminated water. In order for the two processes to be successful, they must be separate in time and in setting. If this requirement isn't met, it is impossible to separate the air from the water once they have mixed.
In a cyclone system with a tangent inlet nozzle, a liquid may also be used to improve the quality of cleaning. Surfactants may be added to the liquid in order to further improve the quality of cleaning. Therefore, the quality of wetting the dust particles may improve, however, this improvement is not due to the high speed of the air flow directed toward the liquid where the high speed of the flow is necessary for the centrifugal separation, but because of the addition of surfactants. And while this may be an effective method of cleaning, the dispersion of water by air flow moving at a high speed makes its later separation more difficult. Also, in introducing the air flow under the liquid contained in the chamber, the aerodynamic resistance enlarges. Furthermore, the filter becomes less reliable because when the chamber is tilted, water can leak into the inlet nozzle. See, for example, U.S. Pat. No. 3,234,713 to Harper et al.
In most known scrubber household vacuum cleaners, the separation of air from water takes place due to the air-water mixture's contact with various hard surfaces such as, the inner walls of the filter, various baffles, etc. See, for example, U.S. Pat. Nos. 1,363,859; 2,306,212; 4,251,241; 4,547,206; 4,874,404; 5,199,963; 5,354,347; and 5,776,215. In this system, as the water wets the surfaces, it remains on the surfaces. This mechanism of separation is ineffective because the contact of the entire air flow with a hard surface cannot be guaranteed. Also, there is no separation of the flow of the contaminated water from the flow of the clean air. Thus, a repeated fusion of the air and water may occur. This is also the direct cause of the ineffective separation of air from water in the turbine in U.S. Pat. No. 3,234,713 to Harper et al. The turbine separated water gathers on the wall of the outlet nozzle and is again caught by the air flow moving along the wall.